2)To reach an absolute pressure of 3atm, you must dive down (in water) to depth of approximately 20m. Why might someone think that the answer was 30m

Someone might think the answer is 30 m when the person cannot distinguish between absolute pressure and gauge pressure.

Explanation:

Given absolute pressure at a depth of 20 m = 3 atm.

When you drive down 30 m in water, the gauge pressure experienced is calculated as;

P = ρgh

Where;

ρ is density of water = 1000 kg/m³

g is acceleration due to gravity, = 9.8 m/s²

h is the depth of water = 30 m

P = (1000)(9.8)(30)

P = 294000 Pa

in atm = ≅ 3 atm.

Thus, someone might think the answer is 30 m when the person cannot distinguish between absolute pressure and gauge pressure.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.